Question

why we say evaporation is when occur below its boiling point

Answer 1

because the upper layer starts evaporating as its temperature is higher than the lower temperature

Answer 2

Evaporation occurs when particles in a liquid pass directly into the gas phase, at a temperature below the boiling point of the liquid. ... If a gas is cooled, its particles will eventually stop moving about fast and form a liquid. This is called condensation and occurs at the same temperature as boiling.